1. Field of the Invention
The present invention relates to a battery and, more particularly, to a sealed battery which is designed to cut-off the flow of current and exhaust internal gas when an internal pressure is increased above an allowable level, and to prevent short-circuit when the battery is deformed by outer shock, thereby enhancing safety thereof.
2. Description of the Related Art
U.S. Pat. No. 4,943,497 discloses a cylindrical sealed battery having a safety device which is designed to cut-off the flow of a current when an internal pressure is increased above an allowable level.
That is, when the internal pressure of the battery is increased above an allowable level, an explosion-proof valve is deformed by the increased pressure so that an electrode lead welded on a projection of the explosion-proof valve is broken by a stripper, thereby cutting-off the flow of current to enhance the safety of the battery.
However, although the explosion-proof device is normally operated when the internal pressure is quickly increased, but not properly operated when the internal pressure is slowly increased since the electrode lead moves following the explosion-proof valve, deteriorating the safety of the battery.
U.S. Pat. No. 5,418,082 discloses another sealed battery having means for cutting-off the flow of a current.
The current cut-off means disclosed in the patent is designed such that a cut-off pressure is determined by a welding state between a projection of a safety valve and a metal plate. However, since the welding state is determined by various parameters such as energy, amplitude, welding pressure, welding time, welding material, etc., the welding state is not always the same.
As a result, the operation of the current cut-off means is not constant. In addition, a welding portion between the projection and the metal plate is oxidized or weakened as the time goes, the same is easily broken by outer force, resulting in the deterioration of the safety of the battery.
Furthermore, the projection of the safety valve is extended downward, the safety valve may urge a power generating member to contact a negative electrode when outer shock is applied to the battery, causing short-circuit.
Therefore, the present invention has been made in an effort to solve the above described problems.
It is an objective of the present invention to provide a sealed battery which is designed to cut-off the flow of current and exhaust internal gas when an internal pressure is increased above an allowable level, and to prevent short-circuit when the battery is deformed by outer shock, thereby enhancing safety thereof.
To achieve the above objective, the present invention provides a sealed battery comprising a can for receiving an electric generator, a shape-variable plate crimped on an opening of the can, a gasket disposed between the can and the shape-variable plate, a fixing plate disposed under the shape-variable plate, an insulating member disposed between the shape-variable plate and the fixing plate, and a projection extending from the fixing plate and welded on the shape-variable plate, the projection being provided with a mechanically weak portion which is designed to be broken at an internal pressure lower than that for separating a welding portion of the projection and the shape-variable plate.
A thickness of the mechanically weak portion is less than those of the shape-variable plate and the fixing plate.
The shape-variable plate is provided with a safety valve which is designed to be broken at an internal pressure higher than that for breaking the mechanically weak portion.
The shape-variable plate comprises two layers attached to each other, and a portion of one of the layer is omitted to define the safety valve.
The insulating member is fixed by a support ring.
An inner diameter of the gasket is less than an outer diameter of the fixing plate.